I've heard mention of using 2 vac ref regs to lower fp at idle.

I'm excluding the stock style reg; only external regs.

Would they be configured in a series on the return side of the TB, or would there be one on both supply and return sides?

 

Thats a good question, I have never heard of dual vacume regulators, but my guess would they would be in line after the TB, I guess they would be able to lower the fp further than just one regulator. I'm starting to look towards a vacume referenced reg. I'm starting to see where 10 more psi would be benificial and where my idle will be impossible to get correct with that much more pressure. I'm running 22 psi now.

 

If one regulator is working properly,
why do you need two?
....
unless you have a 'Hoover Dam' fuel pump,
and the regulator goes out of regulation
at idle.

 

What id like to see is an external regulator with an oversized diaphram on the vacuum side. That way there can be a larger then 1:1 or so reduction in fuel pressure with vacuum. I think some of the FMUs are like that but theyre crazy expensive.

 

I don't see how two regulators would work. Then again I'm not to smart. Pressure would be limmited to the one with the lowest pressure. Maybie if you ran two feed lines and used a switcher like found on a dual tank truck that was hooked up to a rpm related switch. IE one regulator of like 13 to 20 psi and another for 20 to 30 psi. hummm mabie use the shift light to turn on the relay at 3500 or so, might have a lean spot during switching.

 

It all depends on if the regulators are in series or parallel. If they're in parallel the lower fuel pressure regulator will be the pressure at the injectors. If it's in series the pressure at the injectors will be from whichever regulator has the highest pressure.
It doesn't make any sence to me to use 2 regulators but I'm sure if I thought hard enough I could come up with something. Maybe like the ability to adjust fuel pressure using an electric vacuum pump?
The high fuel pressure idle issues is looking rather bleak. I'm not having a problem with my idle and I'm running my 65# at 100# with 28psi gauge. That's at 600rpm and still have control of the AFR.

 

I remember Walt Sherwin mentioning it in a paper he wrote a while back about adjusting TBI fuel pressure.

Just trying to investigate all avenues at this point.

I would think that with a single plane, the idle issues would not be as prevalent as with a dual plane.

As it stands, I'm not having idle issues with 33 psi at idle. Don't really need that much now, just trying to see what I can get away with

With a single plane on a 496, it may not be an issue

 

Well, it just mentions using an external unit after the stock one:

http://www.aces.edu/~gparmer/efi/tbi_adjust.htm

 

Has anyone tried this? Or is there any other way of altering the vacuum signal & therefore the psi.
I ask because I'm having trouble leaning out at light throttle with the vac. hooked up. But w/o vacuum attached I am too rich at idle.

 

I just did a tune for a friend of mine that has a 90 454ss with headers, exhaust, and edelbrock intake manifold. The idle didn't take much to get close enough... it's 7.4L, I don't think it cared what the AFR was .
He has the fuel pressure up to like 14psi but I'm going to hookup my wideband to it and check the WOT runs. You should consider tuning a chip for your truck. It's really not hard to do better with that tune because from what I've seen GM had one of the most conservative tunes on that motor. I'm talking like WAY under tuned with timing. I had to add over 6 degrees everywhere BEFORE he got the headers!
I've used the vacuum regulators in the past and don't like them. I was worried about the high fuel pressure at idle and the low injector on-times but it didn't seem to matter. My 350 has 28psi right now of fuel pressure (around 100#/hr) and she idles with plenty of control.

 

I've tuned alot of trucks here locally for friends and all I can say is that GM really didn't want these things to rely on the knock senosr if they got a bad tank of gas. We're talking 22 deg of timing total on most tunes. Depending on the weight of the vehicle I was able to add 6 to 10 deg of timing. This makes a world of difference for throttle response.

As for fuel control at idle with high fuel pressures. I just got done data logging with my new WB last night and I found out that I'm way lean at idle so I still have plenty of room for adjustment with 70 lb/hr injecotors@21 psi, should be around 100 lb/hr.

 

Yeah, these trucks can use a bunch more timing